1. Field of the Invention
The present invention generally relates to wireless communications networks, such as cellular networks. More particularly, the present invention relates to wireless communication networks comprising relay nodes, generally intended to increase network capacity and extend radio coverage.
2. Overview of the Related Art
Evolution of wireless communication networks has experimented a significant growth in terms of spread and performance, and has recently brought to 3GPP LTE-Advanced (“Third Generation Partnership Project Long Term Evolution Advanced”) standard, which represents a major advance in cellular technology, as being designed to meet needs for high-speed data and media transport as well as high-quality voice and video communications support into the next decade.
More particularly, the 3GPP LTE-Advanced is a standard capable of conveying data between a fixed-location transceiver radiating electromagnetic or radio waves over a respective land area called network cell (which is delimited by electromagnetic radiating power of the of radio wave itself, and often drawn as a hexagon for convention), and typically referred to as Donor eNodeB (DeNB), and User Equipments (UEs, e.g., user terminals, such as cellular phones) within the network cell.
As known, the 3GPP LTE-Advanced employs some advanced technologies, such as Orthogonal Frequency Division Multiplexing (OFDM) or Multiple Input Multiple Output (MIMO) signal transmission technique. As far as 3GPP LTE-Advanced incorporating OFDM technology is concerned, to which reference will be made in the following by way of example only, and wherein downlink access scheme, based on OFDMA—Orthogonal Frequency Division Multiple Access—, differs from uplink access scheme, based on SC-FDMA—Single Carrier Frequency Division Multiple Access—, the need of developing solutions for providing improved user experience while reducing infrastructure costs has brought to the deployment, within the network cells, of one or more relay nodes (or simply relays) each one generally associated with, and supporting, the DeNB of the corresponding network cell.
In general terms, relay nodes ensure coverage extension of the network cells in which they are used, as well as deployment costs reduction of the same, and may enhance the capacity of the network cell and the effective throughput thereof.
More particularly, the operation of the relay nodes within a given network cell is such that the DeNB of the cell communicates with a UE of a subscriber that at that time requires a service in the same network cell (e.g., voice call) through a selected relay node (for example, the one relay node within the cell that is closest to the UE), thereby possibly allowing the UE to be better served via a two-hop path than a single-hop one (thus overcoming, for instance, possible quality degradation of the transmission channel).
Quality degradation of the transmission channel is a relatively frequent occurrence happening when obstructions are present between the DeNB and the user equipment of the subscriber—as those experienced in the presence of physical barriers (e.g., high shadowing environments, such as indoor locations), or interference signals (for instance, radio frequency signals)—, or in case of an excessive distance therebetween, e.g. as a consequence of a restricted radio coverage with respect to the radio coverage theoretically provided by the DeNB. In fact, accounting all gains and losses between the DeNB and the user equipment, a generic user equipment that is located at cell boundaries could experience poor radio channel conditions, and thus it may often be incapable of communicating at all or by making use of reasonably high data rates.
As it is known, access sub-frames and backhaul sub-frames are used for transmission purposes in the presence of relay nodes. More specifically, the access sub-frames are sent over a set of mutually non-interfering wireless access communication links (in both directions, i.e., downlink and uplink), henceforth access links, for interconnecting the relay nodes to the UEs associated thereto, whereas the backhaul sub-frame is sent over the (single) shared wireless backhaul communication link (in both directions, i.e., downlink and uplink), henceforth backhaul links, through which all the relay nodes communicate with the DeNB.
The backhaul sub-frame is scheduled by the DeNB, which selects which relays transmit (in uplink) or receive (in downlink), and on which radio resources, whereas the access sub-frames are scheduled by each relay node, that selects which UEs can transmit/receive and on which radio resources. Hereafter, it will be exemplary assumed (for ease of explanation) that UEs can only be associated to relays nodes.
Being unable to perform reception and transmission operations at the same time using the same frequencies, which would result in strong interference, relay nodes are physically subject to the constraint of having to activate the access link and the backhaul link over separate radio resources. Such physical limitation, frequently referred to as “relay duplexing problem”, requires that access and backhaul link activations be scheduled so as to avoid time-frequency overlapping that could generate high interference levels. In this respect, two different approaches are possible, namely radio resources separation in either the frequency domain (FDD relaying) or the time domain (TDD relaying).
In the FDD relaying approach, access and backhaul sub-frames can be scheduled simultaneously, since signals transmission and reception are carried out using two different frequency intervals. In the TDD relaying approach, instead, access and backhaul sub-frames are assigned to separate time resources with the full frequency spectrum available for each one of them. In this way, both the transmitter and the receiver can operate on a same single frequency, but allocating different time slots for transmission and reception (i.e., transmission and reception signals share the same full frequency spectrum channel, and are spaced apart by multiplexing them on a time basis).
As TDD relaying may better exploit frequency diversity than FDD relaying (due to its using the full spectrum), the former has been deemed as best suited for traffic applications such as Internet or other services. For this reason, the TDD relaying approach has been acknowledged by the present LTE Advanced standard release 10, and to it reference will be exemplarily made in the rest of this document.
In a TDD relaying approach, the decisions of activating, for each relay node, the access links or the backhaul link (referred to as duplexing decision or link scheduling decision in the following) are updated every predetermined time interval, also referred to as duplexing pattern refresh interval (or simply refresh interval), whereas, regardless of the time extension of such refresh interval, the radio resources are allocated within data sub-frames, each one having a time extension of 1 TTI (Transmission Time Interval).
In particular, according to a semi-static link scheduling approach, the duplexing decisions of whether to activate (the uplink and downlink of) the access link or of the backhaul link of a determined relay node are taken at longer time intervals (typically, with the refresh interval of the order of ten TTI), since the frame patterns, communicated through RRC (Radio Resource Control) level signaling, decide in which sub-frames the backhaul link and the access links are activated, whereas in a dynamic link scheduling approach the link scheduling decisions are taken at each sub-frame (with the refresh interval that is equal to 1 TTI).
In the state of the art, solutions are known that provide for link scheduling schemes.
In deliverable D 3.5.3 “Final assessment of relaying concepts for all CGs scenarios under consideration of related WINNER L1 and L2 protocol functions” of WINNER II IST-4-027756 project, 2007, it is shown that the relay solution outperforms the base-stations-only deployment. The relay concept is also applied to some different topological scenarios and it is shown that relay nodes are a cost-effective solution, which provide a high service level. Moreover the FDD and TDD schemes are presented as possible radio resource partitioning solutions.
In R. Schoenen, W. Zirwas, and B. Walke, “Capacity and coverage analysis of a 3GPP-LTE multihop deployment scenario,” in Communications Workshops, 2008. ICC Workshops 08. IEEE International Conference on, pp. 31-36, IEEE, May 2008, the relay performance for coverage and capacity is analyzed in a real urban scenario. The document shows the benefits of relay deployment to extend the cell coverage and increase the spectral efficiency.
In R. Schoenen, R. Halfmann, and B. H. Walke, “An FDD Multihop Cellular Network for 3GPP-LTE,” VTC Spring 2008—IEEE Vehicular Technology Conference, pp. 1990-1994, May 2008, the authors present a performance analysis for relay nodes, for both coverage and capacity, using the TDD scheme. More particularly a pattern for link scheduling which alternates access and backhaul transmissions is defined. Odd sub-frames are dedicated to “hop-1” (i.e., directly connected to the DeNB) users and the relay nodes, while even sub-frames are dedicated to “hop-2” users, i.e., those connected to the relay nodes.
In M. Kaneko and P. Popovski, “Radio resource allocation algorithm for relay-aided cellular OFDMA system,” in ICC'07. IEEE International Conference on Communications, 2007, pp. 4831-4836, IEEE, 2007, the authors propose a Max C/I-based resource scheduling algorithm for a relay enhanced cell with the OFDMA technology. The access and backhaul interleaving is initially as in the previous cited solution, thus the access and backhaul links are alternated: one sub-frame for the backhaul and one sub-frame for the access. A time adaptation algorithm is then introduced: one sub-frame is subtracted from the backhaul and assigned to the access (or vice versa) if this choice produces a throughput increment. The performance of the proposed algorithm is then compared with that of a (theoretical, but unimplementable) upper bound algorithm.
In W. Nam, W. Chang, S. Chung, and Y. Lee, “Transmit optimization for relay-based cellular OFDMA systems,” in ICC'07. IEEE International Conference on Communications, 2007, pp. 5714-5719, IEEE, 2007, two resource allocations problems are formulated for an OFDMA system with relay nodes. The first one uses a fixed power assignment for each subcarrier, the second one proposes a joint power and subcarrier allocation. The interleaving of access and backhaul is the same as in R. Schoenen, R. Halfmann, and B. H. Walke solution, with alternating frames assigned to base stations and relay nodes.
In C.-Y. Hong, A.-C. Pang, and P.-C. Hsiu, “Approximation algorithms for a link scheduling problem in wireless relay networks with QoS guarantee”, IEEE Transactions on Mobile Computing, vol. 9, pp. 1732-1748, 2010, a scheduling algorithm for bandwidth and delay guarantee is investigated in a wireless network using relay. The scheduling algorithm operates on a frame-by-frame basis and it is designed to support Quality of Service (QoS) and real-time services. The proposed algorithm provides a schedule assignment for traffic flows by determining the frame in which the transmission of a subset of flows should occur. This is done by ordering the traffic flows using an EDD (Earliest Due Date) rule.